Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/38—Processing data, e.g. for analysis, for interpretation, for correction
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
  • G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
  • G01V3/26—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with magnetic or electric fields produced or modified either by the surrounding earth formation or by the detecting device
  • G01V3/28—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with magnetic or electric fields produced or modified either by the surrounding earth formation or by the detecting device using induction coils

Definitions

• the invention is related generally to the field of interpretation of measurements made by well logging instruments for the purpose of determining the properties of earth formations. More specifically, the invention is related to a method for determination of anisotropic formation resistivity in a deviated wellbore using multifrequency, multicomponent resistivity data.
• Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) ofthe formation that when properly interpreted are diagnostic ofthe petrophysical properties ofthe formation and the fluids therein.
• a limitation to the electromagnetic induction resistivity well logging instruments known in the art is that they typically include transmitter coils and receiver coils wound so that the magnetic moments of these coils are substantially parallel only to the axis ofthe instrument. Eddy currents are induced in the earth formations from the magnetic field generated by the transmitter coil, and in the induction instruments known in the art these eddy currents tend to flow in ground loops which are substantially perpendicular to the axis ofthe instrument. Noltages are then induced in the receiver coils related to the magnitude ofthe eddy currents. Certain earth formations, however, consist of thin layers of electrically conductive materials interleaved with thin layers of substantially non-conductive material.
• the response ofthe typical electromagnetic induction resistivity well logging instrument will be largely dependent on the conductivity ofthe conductive layers when the layers are substantially parallel to the flow path ofthe eddy currents.
• the substantially non- conductive layers will contribute only a small amount to the overall response ofthe instrument and therefore their presence will typically be masked by the presence of the conductive layers.
• the non-conductive layers are the ones that are typically hydrocarbon bearing and are ofthe most interest to the instrument user. Interpreting a well log made using the electromagnetic induction resistivity well logging instruments known in the art therefore may overlook some earth formations that might be of commercial interest.
• Rosthal U.S. Patent No. 5,329,448 discloses a method for determining the horizontal and vertical conductivities from a propagation or induction well logging device. The method assumes the angle between the borehole axis and the normal to the bedding plane, is known. Conductivity estimates are obtained by two methods. The first method measures the attenuation ofthe amplitude ofthe received signal between two receivers and derives a first estimate of conductivity from this attenuation. The second method measures the phase difference between the received signals at two receivers and derives a second estimate of conductivity from this phase shift. Two estimates are used to give the starting estimate of a conductivity model and based on this model, an attenuation and a phase shift for the two receivers are calculated. An iterative scheme is then used to update the initial conductivity model until a good match is obtained between the model output and the actual measured attenuation and phase shift.
• Hagiwara (U.S. Patent No. 5,656,930) shows that the log response of an induction-type logging tool can be described by an equation ofthe form
• V oc (- 2e ikL (1 - ⁇ kV) + ikl(e ik ⁇ - e ikL )) (1)
• k 2 ⁇ 2 ⁇ ( ⁇ h + i ⁇ h I ⁇ ) (3)
• L is the spacing between the transmitter and receiver
• k is the wavenumber of the electromagnetic wave
• ⁇ is the magnetic permeability ofthe medium
• ⁇ ? is the deviation ofthe borehole angle from the normal to the formation
• ⁇ is the anisotropy factor for the formation
• ⁇ is the angular frequency ofthe electromagnetic wave
• ⁇ h is the horizontal conductivity ofthe medium
• e h is the horizontal dielectric constant ofthe medium.
• Eq. (3) is actually a pair of equations, one corresponding to the real part and one corresponding to the imaginary part ofthe measured signal, and has two unknowns.
• the two needed measurements can be obtained from (1) R and X signals from induction logs, (2) phase and attenuation measurements from induction tools, (3) phase or attenuation measurements from induction tools with two different spacings, or (4) resistivity measurements at two different frequencies.
• e can be neglected in Eq. (3) and from known values of ⁇ and ⁇ , the conductivity ⁇ can be determined from k, assuming a value of// equal to the permittivity of free space.
• Wu U.S. Patent 6,092,024 recognized that the solution to eqs. (l)-(3) may be non-unique and showed how this ambiguity in the solution may be resolved using a plurality of measurements obtained from multiple spacings and/or multiple frequencies.
• Tabarovsky and Epov also describe a method of substantially reducing the effect on the voltage induced in transverse receiver coils which would be caused by eddy currents flowing in the wellbore and invaded zone.
• the wellbore is typically filled with a conductive fluid known as drilling mud. Eddy currents that flow in the drilling mud can substantially affect the magnitude of voltages induced in the transverse receiver coils.
• the wellbore signal reduction method described by Tabarovsky and Epov can be described as "frequency focusing", whereby induction voltage measurements are made at more than one frequency, and the signals induced in the transverse receiver coils are combined in a manner so that the effects of eddy currents flowing within certain geometries, such as the wellbore and invasion zone, can be substantially eliminated from the final result.
• Tabarovsky andEpov do not suggest any configuration of signal processing circuitry which could perform the frequency focusing method suggested in their paper.
• Strack et al. (U.S. Patent 6,147,496) describe a multicomponent logging tool comprising a pair of 3-component transmitters and a pair of 3-component receivers. Using measurements made at two different frequencies, a combined signal is generated having a reduced dependency on the electrical conductivity in the wellbore region.
• U.S. Patent 5, 781,436 to Forgang et al discloses a suitable hardware configuration for multifrequency, multicomponent induction logging.
• the electromagnetic induction signals are measured using first receivers each having a magnetic moment parallel to one ofthe orthogonal axes and using second receivers each having a magnetic moment perpendicular to a one ofthe orthogonal axes which is also perpendicular to the instrument axis.
• a relative angle of rotation ofthe perpendicular one ofthe orthogonal axes is calculated from the receiver signals measured perpendicular to the instrument axis.
• An intermediate measurement tensor is calculated by rotating magnitudes ofthe receiver signals through a negative ofthe angle of rotation.
• a relative angle of inclination of one of the orthogonal axes that is parallel to the axis ofthe instrument is calculated, from the rotated magnitudes, with respect to a direction ofthe vertical conductivity. The rotated magnitudes are rotated through a negative of the angle of inclination.
• Horizontal conductivity is calculated from the magnitudes ofthe receiver signals after the second step of rotation.
• An anisotropy parameter is calculated from the receiver signal magnitudes after the second step of rotation.
• Vertical conductivity is calculated from the horizontal conductivity and the anisotropy parameter.
• Co-pending United States Patent Application Ser. No. 09/825,104 (referred to hereafter as the '104 application) filed on April 3, 2001 teaches a computationally fast method of determination of horizontal and vertical conductivities of subsurface formations using a combination of data acquired with a transverse induction logging tool such as the 3DEX SM tool and data acquired with a conventional high definition induction logging tool (HDIL).
• the data are acquired in a vertical borehole.
• the HDIL data are used to determine horizontal resistivities that are used in an isotropic model to obtain expected values ofthe transverse components ofthe 3DEX SM .
• the data are transformed to a subsurface formation coordinate system.
• Multifrequency focusing is applied to the measurements at a plurality of frequencies.
• Horizontal formation conductivities are determined from a subset ofthe focused conductivity measurements.
• Vertical formation conductivities are determined from the focused conductivity measurements associated with the subsurface formation and the horizontal conductivities.
• a transformation independent of the formation azimuth may be used to determine the conductivity ofthe transversely anisotropic formation.
• FIG. l shows an induction instrument disposed in a wellbore penetrating earth formations.
• FIG.2 shows the arrangement of transmitter and receiver coils in a preferred embodiment ofthe present invention marketed under the name 3DEX SM
• FIG.3 shows an earth model example used in the present invention.
• FIG. 4 is a flow chart illustrating steps comprising the present invention.
• FIG. 1 shows an induction well logging instrument 10 disposed in a wellbore 2 penetrating earth formations.
• the earth formations are shown generally at 6, 8, 12 and 14.
• the instrument 10 is typically lowered into the wellbore 2 at one end of an armored electrical cable 22, by means of a winch 28 or similar device known in the art.
• An induction well logging instrument which will generate appropriate types of signals for performing the process ofthe present invention is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al.
• the prior art induction logging tool includes a transmitter coil and a plurality of receiver coils 18A-18F.
• the coils in the prior art device are oriented with the coil axes parallel to the axis ofthe tool and to the wellbore.
• FIG.2 the configuration of transmitter and receiver coils in a preferred embodiment ofthe 3DExplorer TM induction logging instrument of Baker Hughes is disclosed.
• a logging instrument is an example of a transverse induction logging tool.
• Three orthogonal transmitters 101, 103 and 105 that are referred to as the T x , T z , and T y transmitters are shown (the z- axis is the longitudinal axis ofthe tool).
• the transmitters 101, 103 and 105 are receivers 107, 109 and 111, referred to as the R ⁇ , P-, and R y receivers, for measuring the corresponding components (H ⁇ , H ⁇ , H ⁇ of induced signals.
• cross- components are also measured. These are denoted by H ⁇ , H ⁇ etc.
• Fig.3 is a schematic illustration ofthe model used in the present invention.
• the subsurface ofthe earth is characterized by a plurality of layers 201a, 201b, . . ., 201i.
• the layers have thicknesses denoted by h l s h 2 , . . . h,.
• the horizontal and vertical resistivities in the layers are denoted by R hl , R h2 , . . .R h , and R , R v2 , . . .R v , respectively.
• the model may be defined in terms of conductivities (reciprocal of resistivity).
• the borehole is indicated by 202 and associated with each ofthe layers are invaded zones in the vicinity ofthe borehole wherein borehole fluid has invaded the formation and altered is properties so that the electrical properties are not the same as in the uninvaded portion ofthe formation.
• the invaded zones have lengths L x0l , L x02 , . . .L x0i extending away from the borehole.
• the resistivities in the invaded zones are altered to values R x0] , R x02 , . . .R x0j .
• the invaded zones are assumed tp be isotropic while an alternate embodiment ofthe invention includes invaded zones that are anisotropic, i.e., they have different horizontal and vertical resistivities.
• the discussion ofthe invention herein may be made in terms of resistivities or conductivities (the reciprocal of resistivity).
• the z- component ofthe 3DEX SM tool is oriented along the borehole axis and makes an angle #(not shown) with the normal to the bedding plane.
• the x- component ofthe tool makes an angle ⁇ with the "up" direction.
• the number m of frequencies ⁇ is ten.
• n is the number of terms in the Taylor series expansion. This can be any number less than or equal to m.
• the coefficient s 3/2 ofthe ⁇ 3/2 term ( ⁇ being the square of k, the wave number) is generated by the primary field and is relatively unaffected by any inhomogeneities in the medium surround the logging instrument, i.e., it is responsive primarily to the formation parameters and not to the borehole and invasion zone. In fact, the coefficient s 3/2 ofthe ⁇ ) 3/2 term is responsive to the formation parameters as though there were no borehole in the formation.
• the H ⁇ and H ⁇ are applied to the H ⁇ and H ⁇ , components.
• the H ⁇ and H ⁇ would be the same, with both being indicative ofthe vertical conductivity ofthe formation.
• the sum ofthe H ⁇ and H w is used so as to improve the signal to noise ratio (SNR).
• SNR signal to noise ratio
• This multifrequency focused measurement is equivalent to the zero frequency value.
• the zero frequency value may also be obtained by other methods, such as by focusing using focusing electrodes in a suitable device.
• the method ofthe '104 application also uses data from a prior art High Definition Induction Logging (HDIL) tool having transmitter and receiver coils aligned along the axis ofthe tool.
• HDIL High Definition Induction Logging
• These data are inverted using a method such as that taught by Tabarovsky et al, or by U.S. Patent 5,884,227 to Rabinovich et al., the contents of which are fully incorporated herein by reference, to give an isotropic model ofthe subsurface formation.
• a focusing method may also be used to derive the initial model.
• the HDIL tool is responsive primarily to the horizontal conductivity ofthe earth formations when run in a borehole that is substantially orthogonal to the bedding planes.
• the inversion methods taught by Tabarovsky et al and by Rabinovich et al are computationally fast and may be implemented in real time. These inversions give an isotropic model ofthe horizontal conductivities (or resistivities)
• a forward modeling is used in the ' 104 application to calculate a synthetic response ofthe 3DEX TM tool at a plurality of frequencies.
• a suitable forward modeling program for the purpose is disclosed in Tabarovsky and Epov "Alternating Electromagnetic Field in an Anisotropic Layered Medium" Geol. Geoph.. No. 1. pp. 101-109. (1977). Multifrequency focusing is then applied to these synthetic data .
• the method taught by Tabarovsky is used for the purpose.
• the output from model output should be identical to the multifrequency focused measurements.
• the anisotropy factor ⁇ is then calculated in the ' 104 application.
• ⁇ t is the conductivity obtained from the HDIL data, i.e., the horizontal conductivity.
• the vertical conductivity is obtained by dividing ⁇ t by the anisotropy factor from eq. (5).
• An important aspect ofthe '104 application is that in a vertical borehole, the measurements made by a HDIL tool depend only on the horizontal conductivities and not on the vertical resistivities.
• the method ofthe invention disclosed there is to obtain an isotropic model from the HDIL data, use the isotropic model to predict measurements made on other components and to use a difference between the predicted and actual measurements to obtain the vertical conductivity.
• the method ofthe present invention can be viewed as finding a combination of 3DEX S measurements (called modes ofthe induction measurements) that are responsive only to the horizontal conductivity, deriving a model of horizontal conductivity from this combination of measurements, predicting values of other components of 3DEX SM measurements and using a difference between these predicted measurements and the actual measurements to determine a vertical conductivity.
• the desired combination includes only the principal component measurements, i.e., upon H ⁇ ,
• the method ofthe present invention starts with an estimate ofthe dip and azimuth ofthe formation relative to the borehole axis 300. These angles are defined below, hi addition, a sensor on the logging tool also provides another angle measurement called the "toolface angle" that is also used in the analysis ofthe data.
• Multifrequency focused data are derived from multifrequency measurements 301. The data are transformed 303 as discussed below to give measurements that are indicative only of horizontal conductivity. These data are inverted 305 to give a model ofthe horizontal conductivity ofthe data. These estimates of horizontal conductivity are used in an isotropic model as estimates ofthe vertical conductivity 307. The measured data are then inverted using this initial estimate of vertical conductivities 309.
• H ⁇ the matrix of magnetic components
• the matrix, Hj is symmetric.
• the non-diagonal elements are not needed in the present invention.
• the formation resistivity is described as a tensor, p.
• the resistivity tensor has only diagonal elements in the absence of azimuthal anisotropy:
• the "tool coordinate” system (7- 2- 3-) can be obtained from the "formation coordinate” system (x-, v- z-) as a result of two sequential rotations:
• the second rotation is described using matrices ⁇ and ⁇ 1
• H M the formation coordinate system
• H ⁇ the tool coordinate system
• Equation (13) Taking into account Equations (11), (12), (14) and (15), we can re-write Equation (13) as follows:
• Equation (18) Taking into account all the above calculations, we can represent Equation (18) in the following form:
• the method ofthe present invention involves defining a linear combination ofthe measurements that are responsive substantially to the horizontal conductivity and not responsive to the vertical conductivity.
• the linear combination is that of measurements h n , h 22 , and h 33 (i.e., the principal components only), although in alternate embodiments ofthe invention, a linear combination of any ofthe measurements may be used.
• the example given below is that ofthe preferred embodiment.
• h 22 C, 2 SjA ⁇ -2C ⁇ S ⁇ S ⁇ 2 h xz +S ⁇ C ⁇ S ⁇ h yz +S ⁇ 2 S ⁇ 2 h zz +C ⁇ 2 h yy +S e C f S.h-,
• Equations (13)-(15) may be represented in the following form:
• Equation (23) A linear combination of Equations (23) is defined in the form:
• Equation (25) [0039] Detailed consideration of Equation (25) yields:
• the method ofthe present invention involves defining the coefficients, a and ⁇ , in such a way that the resulting linear combination, h, does not depend on the vertical resistivity. To achieve that, we need to null the following part ofthe expression for h:
• Equation (29) Equation (29)
• Equation (25) may be presented in the form:
• the conductivities may be derived.
• a difference between the model output and the measured values may then be used in the iterative procedure described with respect to Fig. 4.
• Multifrequency Focused (MFF) data is a linear combination of single frequency measurements so that the derivation given above is equally applicable to MFF data. It can be proven that the three principle 3DEXTM measurements, MFF (multi-frequency focusing) processed, may be expressed in the following form:
• Equation 39 The matrix coefficients of Equation 39 depend on ⁇ note ⁇ r , and three trajectory measurements: deviation, azimuth and rotation.
• Equation 39 The components ofthe vector in the right hand side of Equation 39 represent all non-zero field components generated by three orthogonal induction transmitters in the coordinate system associated with the formation. Only two of them depend on vertical resistivity: h ⁇ and h w . This allows us to build a linear combination of measurements, h n , h 22 , and h 33 in such a way that the resulting transformation depends only on h. z and h xz , or, in other words, only on horizontal resistivity.
• T be the transformation with coefficients a, ⁇ and ⁇ :
• T 0 (h n + h 22 ) sin 2 ⁇ - h 33 (1 + cos 2 ⁇ ) (42)
• the present invention has been discussed above with respect to measurements made by a transverse induction-logging tool conveyed on a wireline. This is not intended to be a limitation and the method is equally applicable to measurements made using a comparable tool conveyed on a measurement- while-drilling (MWD) assembly or on coiled tubing.
• MWD measurement- while-drilling

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• 2002
  • 2002-02-07 US US10/072,173 patent/US6636045B2/en not_active Expired - Lifetime
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